Re: [PATCH] sched/fair: no sync wakeup from interrupt context

[Libo Chen]

On 8/3/22 02:18, Ingo Molnar wrote:
> * Libo Chen <libo.chen@xxxxxxxxxx> wrote:
>
> > On 8/1/22 06:26, Ingo Molnar wrote:
> > > * K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
> > >
> > > > Hello Libo and Peter,
> > > >
> > > > tl;dr
> > > >
> > > > - We observed a major regression with tbench when testing the latest tip
> > > >     sched/core at:
> > > >     commit 14b3f2d9ee8d "sched/fair: Disallow sync wakeup from interrupt context"
> > > >     Reason for the regression are the fewer affine wakeups that leaves the
> > > >     client farther away from the data it needs to consume next primed in the
> > > >     waker's LLC.
> > > >     Such regressions can be expected from tasks that use sockets to communicate
> > > >     significant amount of data especially on system with multiple LLCs.
> > > >
> > > > - Other benchmarks have a comparable behavior to the tip at previous commit
> > > >     commit : 91caa5ae2424 "sched/core: Fix the bug that task won't enqueue
> > > >     into core tree when update cookie"
> > > >
> > > > I'll leave more details below.
> > > Mel Gorman also warned about this negative side-effect in:
> > >
> > >      Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context
> > >      Date: Fri, 15 Jul 2022 11:07:38 +0100
> > >      Message-ID: <20220715100738.GD3493@xxxxxxx>
> > >
> > >      https://urldefense.com/v3/__https://lore.kernel.org/all/20220715100738.GD3493@xxxxxxx/__;!!ACWV5N9M2RV99hQ!PQsIeuK0UwII-A0xS-B3plepNniNeyw14OJowT1cYL-tnuN99MkWfg9C8P60tVFFrnxj0NEanUmEkA$
> > ?? Mel was talking about a completely different thing, I brought up a
> > different patch that I wanted to revert and Mel thought it would hurt other
> > workloads which don't benefit from pulling but
> > as you can see, tbench somehow benefits from it, at least according to one
> > metric from one workload.
> Yeah - but nevertheless the discussion with Mel was open-ended AFAICS, and
> the 'major tbench regression' report by K Prateek Nayak above still stands
> and needs to be investigated/understood, right?
Oh yes, I have no issue with holding the patch back until the regression 
is fully understood. I was just a little confused on your reference to 
Mel's comments. Anyway, I will post my investigation soon.


Libo
> Thanks,
>
> 	Ingo